1. Field of the Invention
This invention relates to integrated circuit antifuse devices which provide an electrically programmable connection between two unconnected points of a circuit. More specifically, the present invention relates to the use of a ferroelectric material in the antifuse elements of such antifuse programmable array devices.
2. Related art
This application relates to co-pending patent application Ser. No. 08/249,524 filed herewith, hereby incorporated by reference, and assigned to Symetrix Corporation.
3. Brief Description of the Prior Art
It is known to provide programmable integrated circuits which permit a user to customize the application of the circuit by programming ("blowing") electronic interconnections within the circuit to fulfill a specific application need. Integrated circuits such as Programmable Array Logic (PALs), Programmable Logic Devices (PLDs), Field Programmable Gate Arrays (FPGAs), and Programmable Read-Only Memories (PROM's) are typical of such programmable devices. These devices provide arrays of programmable elements which a user may elect to connect or disconnect by electrical means. "Fusible" devices are manufactured with each element initially connected and are programmed by applying sufficient current to an element to "blow" the fuse thereby disconnecting the element. "Antifuse" devices are manufactured with each element initially unconnected and are programmed by applying an electrical field across an element sufficient to cause a breakdown of an insulating material within an element. The breakdown of the insulating layer forms a conductive filament connecting the two previously unconnected points of the element.
A large class of such devices are one-time programmable (OTP) devices in that each element may be programmed only once to meet the requirements of a particular application. Once programmed, the element cannot be changed again. Most antifuse devices use a dielectric layer of material between the unconnected contacts to provide the electrical insulation between the contacts. In an electric field of sufficient strength, the dielectric material heats to the point of breakdown and forms a conductive filament through an aperture in the dielectric material. This conductive filament connects the two previously unconnected contacts of the programmable antifuse element.
Basire et al. in U.S. Pat. No. 4,488,262 (issued Dec. 11, 1984) teach the design of a PROM device implemented with an array of antifuse elements. Basire et al. disclose a device embodying a two dimensional array of antifuse elements with intersecting row and column electrodes disposed across the insulative layer of each antifuse element. An electric field is controllably applied across each antifuse element to be programmed. The field is applied to the row electrode and column electrode which intersect at the antifuse element to be programmed. A sufficiently large electric field breaks down the insulative layer between the two intersecting electrodes to form a conductive filament through the insulative layer.
It is desirable to use materials for the insulating layer which have high resistivity in the unprogrammed state and yet breakdown to form a conductive filament of low resistivity. However, such materials have presented problems in prior designs. Although it is desirable to maintain a high resistivity in the unprogrammed state, a high dielectric constant of the insulating material in the unprogrammed state requires a correspondingly high DC electric field amplitude to force the breakdown of the insulating material. When the required field amplitude rises too high for practical applications, some prior designs have taught the use of dopants applied to the electrodes or the insulating layer to improve conductivity. Hamdy et al. teach the use of arsenic dopants applied to one or both electrodes in U.S. Pat. No. 4,889,205 (issued Feb. 6, 1990). The arsenic is intended to reduce the resistivity of the conductive filament formed by the breakdown of the insulative layer. Similarly, Lee teaches in U.S. Pat. No. 5,250,459 (issued Oct. 5, 1993) the use of antimony as a dopant to be applied to one or both electrodes. The antimony flows into the conductive filament when formed by the breakdown of the insulative layer. However, these approaches typically alter both the programmed and unprogrammed resistivity.
Basire et al., at first reading, teach that a solution to the above problems is to use materials with a low dielectric constant, such as "transition metal oxides or titanate such as tantalum oxide, vanadium oxide, zirconium oxide, niobium oxide, barium titanate and strontium titanate." However, on closer analysis, some of the materials named by Basire et al., such as barium titanate and strontium titanate, are known to have high dielectric constants so they do not accurately reflect the desired properties as taught by the patent. Barium titanate and strontium titanate are ferroelectric materials, but the ferroelectric nature of the materials is not recognized in the teachings in Basire et al.
Another problem in prior designs arises from their use of silicon oxide or other similar materials to form the dielectric layer of each antifuse element. These materials are broken down into conductive material but then re-oxidize over time decreasing their conductivity eventually causing failure of the programmed antifuse element.
In addition, the minimum size of a programmed antifuse element constructed using a single material such as silicon oxide as the dielectric material is determined by the resistivity properties of silicon oxide when it is broken down to form a silicon conductive filament. Achieving a resulting resistance in a fused conductive filament low enough for high speed applications of the connected circuit requires a relatively large cross-sectional area of the conductive filament. The large cross-sectional area of the fused conductive filament determines the lower bound of the size of each fusible element and therefore the maximum density of fusible elements within an integrated circuit. Wong et al., in U.S. Pat. No. 5,250,464 (issued Oct. 5, 1993), teach a new method of constructing an antifuse element which reduces the surface area required for each element so as to increase the density of elements in a single programmable array device. However, the lower bound for the size of the antifuse element as taught by Wong et al. remains limited by the use of a dielectric material such as silicon oxide.
Solution to the Problem
The present invention teaches the construction of an antifuse element which uses ferroelectric materials in the dielectric layer. Ferroelectric materials have very high dielectric constants and so provide an excellent insulating layer between the two unconnected electrodes in the programmable antifuse elements of the present invention. However, when broken down, ferroelectric materials form conductive filaments of very low resistance to permit high speed signal transmission.
A combination of AC and DC electric fields is applied to breakdown the ferroelectric dielectric layer as taught by co-pending U.S. patent application Ser. No. 08/249,524. That is, an AC electric field is first applied across the antifuse element then followed by a DC electric field which causes the ferroelectric material to break down. The high dielectric constant of these materials increases the power dissipated through dielectric heating within the material in response to an applied AC electric field. The combined effects of the AC and DC electric fields breaks down the ferroelectric material more easily than could either field alone.
Additionally, the use of ferroelectric materials in the dielectric layer of the antifuse element of the present invention reduces the problem present in prior designs of oxidation of the fused conductive element of prior designs. Ferroelectric materials break down to form electrically conductive oxide materials and therefore do not change their resistivity over time due to oxidation of the conductive filament. Their resistivity remains relatively constant in comparison to silicon oxide materials used in prior designs.
The present invention also teaches that multiple stacked layers of materials of varying compositions may be used to form the dielectric layer of antifuse elements. The annealing of these dielectric materials caused by the combined application of AC and DC electric fields blends the various materials used in the dielectric layer to form a new material whose resistivity is determined by the materials blended to form the conductive filament. In this manner, a particular resistivity range may be selected in the manufacturing process. The ability to alter the resulting resistivity of the conductive filament also permits the filament cross-sectional size to be reduced by reducing the resistivity of the resultant filament. A reduced filament cross-sectional size enables the manufacturing of higher densities of antifuse elements within an integrated circuit. More generally, each layer of material may be manufactured by substitution of atoms in the lattice structures. Prior designs have recited doping processes to add certain elements to the dielectric or electrode layers of the antifuse elements. Doping allows a limited range of alteration of the electrical properties of the dielectric material. Further, the use of ferroelectric materials in the present invention permits a broader range of alteration of the dielectric material through substitution processes for binding locations within the lattice structure of the ferroelectric material. This broad range of alteration of the dielectric material permits the material to be customized for certain desired electrical and physical properties appropriate to a particular application of the antifuse element.